In typical liquid crystal displays (LCDs), the light-extracting directions of backlight sources are usually constant, and the light-extracting distribution is maintained within some particular range, such as shown in FIG. 1, which shows luminance of a backlight source as a function of angle. The light-extracting direction of a backlight source refers to the general direction in which light is emitted (extracted) from the light source. A light-extracting distribution refers to range of angles over which light is emitted (extracted) from the light source, where the luminance of the light in this range of angles is greater than some threshold. The light-extracting distribution according to FIG. 1 can result in lack of variation of the functions of LCDs, which can reduce display quality (such as for dynamic display) and degrade contrast.
One conventional type of backlight module used with LCDs has a dual-direction light-extracting feature that adds variety to the functions of LCDs. FIG. 2 shows a conventional dual-direction light-extracting backlight module 10, which includes a first light guide set 12 and a second light guide set 14 arranged in a stacked manner (the first light guide set 12 stacked over the second light guide set 14). In addition, an optical film set 24 is stacked over the light guide set 12. The light guide sets 12, 14 respectively include light guide plates 20, 22, light sources 16, 18, and reflection sheets 26, 28. The light source 16 is disposed on one side (left side in FIG. 2) of the light guide plate 20, while the light source 18 is disposed on one side (right side) of the light guide plate 22. Moreover, the reflection sheets 26, 28 are respectively disposed on the opposite sides of corresponding light guide plates 20, 22 (opposite to the sides where the light sources 16, 18 are disposed).
The light guide sets 12, 14 of the backlight module 10 can provide two light-extracting directions (as indicated by the two crossed arrows). However, such a dual-direction light-extracting backlight module 10 has disadvantages including an asymmetric light-extracting distribution, poor transmittance and high manufacturing complexity, and the backlight module 10 cannot provide sufficient display function variety to fulfill various function requirements of existing display devices on the market since only two particular light-extracting direction ranges are provided by the backlight module 10. Furthermore, the light guide set 12 and the light guide set 14 are stacked on one another, so that the light extraction of the underlying light guide set 14 is affected by the light extraction of the overlying light guide set 12.